feat: per-group sort order (defined/alpha) in group form and converter

src/lib/types.ts

@@ -37,6 +37,8 @@ export interface Group {
   baseUnitId: string;
   /** Relative scale for cross-group YOLO conversions. */
   toUniversal: number;
+  /** Display order for units in the converter. Defaults to 'defined'. */
+  sortOrder?: 'defined' | 'alpha';
 }
 
 /** Full data payload for units and groups. */